Significance There a number of developmental disorders such as autism which profoundly effect social interactions and the ability to become a productive member of society. The brain regions that organize normal social behavior are currently unknown. Understanding the neurobiology of social cognition would provide important clues as to which brain systems may be dysfunctional in disorders such as autism. Objectives We evaluate the role of a brain region called the amygdala in regulating normal social behavior in macaque monkeys. Mature macaque monkeys are observed in normal dyadic and tetradic social interactions and during exposure to videotapes of conspecifics and to novel objects. Results Mature macaque monkeys with a dysfunctional amygdala fail to demonstrate normal reluctance to engage other conspecifics or novel objects. Interestingly, normal control animals apparently detect a difference in the animals without amygdala function and respond in a more affiliative manner. The experimental animals thus appear to be both less threatened by and less threatening to normal control animals with which they are interacting. Future Directions We will evaluate the social interactions of the experimental animals in tetradic social encounters. This is a more challenging and dynamic social setting and we predict that the experimental animals will perform inappropriately in this context. We are also developing computer-aided testing procedures in which to evaluate the role of the amygdala in the appreciation of facial expressions. KEYWORDS social behavior, autism, aggression